1. Field of the Invention
The invention relates to a drive device, as described in the preamble of claim 1.
2. The Prior Art
From document WO 2006/101156 A1 a hydraulic drive device for a folding press and a method for operating the latter are known, according to which with an open hydraulic system for supplying operating cylinders a hydraulic pump is driven by a speed-controlled motor. The speed can be varied according to the different requirements for movement operations, such as rapid traverse, press traverse, emergency stop, return stroke, from standstill to maximum speed.
From document JP 2002-147404 A a drive device for a bending press is known comprising a press frame and with a fixed press beam and a press beam adjustable relative to the latter, wherein the drive device consists of a hydraulic system with a hydraulic pump and a ring line with control valves for a switchable flow circuit for a pressure medium for alternately charging two pressure chambers of at least one pressure cylinder for the adjustment of the press beam. The hydraulic system is an open system supplied with pressure medium from a tank, which excludes a pressure store, as the differential volume of medium required for the different filling volumes of the pressure chambers of the hydraulic cylinder is conveyed respectively into or out of the tank. This requires a suitably high overall volume and conveying volume of the pressure medium for the alternate charging of the hydraulic cylinder.
From document DE 1 027 951 B a hydraulic-rapid control is known with devices for avoiding control shocks for hydraulic presses, hydraulically operated machine tools or the like. The rapid control comprises in addition to a main valve an additional control valve, which by means of a connection line produces the connection between a pressure source, e.g. a pump, an accumulator, a press cylinder and the main control valve. Furthermore, it comprises a bypass line, which leads directly from the pressure source to the main control valve, wherein the latter has a smaller cross section than the connection line running via the additional control valve, and in said bypass line if necessary also a throttle valve can be provided. Thus pressure shocks caused by the control processes are counteracted by the corresponding cross sectional selection of the connection line as well as the regulating possibility of the throttle valve and as far as possible pressure equalization is achieved.
From a further document, DE 21 40 183 A1, a hydraulic drive device is known with an open design, with a tank, pump and feed line for charging a pressure chamber of an operating cylinder with a pressure medium. A bypass line of the pressure medium from the additional pressure chamber can be supplied via a switch valve optionally to the feed line or returned as a function of the determined pressure level in the feed line or the pressure chamber via an outflow line into the tank. By mixing the outflowing pressure medium into the feed line with a predefined output of the pump there is an increase in the displacement speed of the press beam before the actual pressing operation, i.e. a rapid approach and thereby a shortened cycle time.
From document EP 0 967 028 A1 a hydraulic press is known with a hydraulic operating cylinder acting on both sides with active faces of varying size on both sides of a piston. The hydraulic drive device comprises a container for the pressure medium, which by means of an electric motor driven pump is fed into in a conveying circuit and via control valves to the operating cylinder, optionally to the pressure chambers separated by the piston for the movement of a press beam connected in motion to the piston or a piston rod for making an adjustment for a press stroke and a return stroke. To reduce the operating cycle between lines for the alternate charging of the pressure chambers of the operating cylinder an additional suction line with an additional suction valve is provided for a rapid equalisation of the volume flows of the pressure chambers with different capacities.
From a further document, AT 008 633 U1, a hydraulic drive unit is known for a press, e.g. a die bending press, for activating a press beam by means of a double-acting hydraulic cylinder with different active surfaces. The hydraulic drive unit comprises a container for a pressure medium, an electric motor driven pump and supply lines and control valves and a flow-connected, chargeable pressure store, by means of which for compensating the inherent mass of the press beam a counteracting force is provided by the operating cylinder.
Conventional driving techniques in presses of higher force categories use a hydraulic load-sensing principle for the operating process and a hydraulically controlled drop of the pressure beam in rapid traverse downwards. As in load-sensing operation a control reserve of the pressure loss is required and the movement is also controlled during the rapid-up or rapid-down traverse via a resistance control, losses are caused as defined by the principle. The resulting oil-warming has to be reduced in many cases by oil coolers. A further disadvantage is that the electric drive motor and the connected hydraulic constant flow pump run during the entire operating period, which results in losses and unnecessary noise. Such a driving configuration comprises one hydraulic supply unit per machine, which in addition to the pump and motor also comprises tank connecting lines and various auxiliary devices and prevents a strictly modular construction, in which each axis is completely separate and compact. Numerous hydraulic connections have to be produced in the assembly process of the press. The latter may lead to leakages caused by permeability, breaks in the tubing or during the replacement of hydraulic components.
Furthermore, today electric-hydraulic hybrid drives are known, the basic idea of which is to connect a hydraulic constant flow pump with a variable-speed electric motor, in order to control the speed electrically in this way and to use the connected hydraulic circuit with a hydraulic cylinder at its end on the one hand for simple force translation and on the other hand as a change-speed gearbox for rapid operation shifting. An open hydraulic system is used for this, which, in addition to the actual operating cylinder, designed as a single-thread differential cylinder, uses an additional plunger cylinder. Its hydraulically active surface is equal to the ring surface of the differential cylinder. The cylinder chambers assigned to both these areas are switched in rapid traverse, such that the action of a through rod cylinder is adjusted which is moved by the pump. The piston-side chamber suctions in rapid traverse from the tank. In working operation the pump acts on the piston side of the differential cylinder, the annular side lies on a hydraulic storage unit for holding.